While tank sensors can be finicky, the fresh water one is usually the least troublesome. Check that all the little wires to the probes in the side of the tank are properly attached and unbroken. (That, of course, requires access to the side of the tank--which isn't always the easiest to get.)
Some motorhomes do have little clearance and a quite poor departure angle. However, it does sound as though you may have a broken or weak spring. I second the advice to get it checked out by a truck spring and chassis shop. Particularly with leaf springs, it's not especially uncommon to have a partial break (one or more leaves broken) that is not immediately obvious due to being hidden in the middle of everything. Bad shocks will not generally result in a low ride height as they do not support the weight (to any great extent, if at all).
Air helper springs are a nice upgrade, to be certain, but of course are not a suitable replacement for a broken spring.